1. Field of the Invention
Embodiments of the invention generally relate to dispersion and adsorption coefficient estimation, and more particularly, to methods for estimating porous-media longitudinal dispersion and adsorption coefficients through an analysis of pressure data during a viscosity switch between two solutions, one of which includes a viscosifying agent having a non-linear concentration-viscosity dependence. In accordance with certain embodiments, methods are also provided for estimating adsorption coefficients.
2. Description of the Related Art
Dispersion of miscible fluid flow in porous media has been investigated as a method for quantifying geological characterization and for defining reservoir heterogeneity for enhanced oil recovery (EOR). Conventional systems demonstrate that pore-scale heterogeneities exhibit local velocity variations. Due to these variations, not all solute particles move at a mean velocity; some move faster while others move slower, generating a distribution of solute spread about the mean velocity. This solute spreading (or smearing) is termed “mechanical” dispersion. Similarly, molecular diffusion also generates some smearing. The combined effect of mechanical dispersion and molecular diffusion is termed “microscopic” dispersion or in the context of a reservoir simulation, “physical” dispersion, to differentiate it from a “numerical” dispersion.
An understanding of dispersion of miscible fluid flow in a porous medium is important because dispersion governs the degree of mixing between different solutions and their respective phases. The degree of mixing is significant for various EOR processes. For surfactant EOR, for example, the degree of mixing governs a degree of emulsification and an associated interfacial tension (IFT) reduction. For polymer EOR, the degree of mixing between different salinity waters will govern the effectiveness of a polymer flood as salinity has a direct impact on polymers viscosibility. Therefore, an operator in the area of EOR would find it important to quantify the magnitude of physical dispersion for the optimization of EOR.
Some conventional dispersion estimation techniques involve the measurement of tracer-concentration smearing either in-situ or at a production outlet. In the former approach, an in-situ measurement, for example, nuclear magnetic resonance (NMR), is conducted, such that the spreading of an injected tracer can be tracked with time. The NMR data is then used to estimate a dispersion coefficient (see FIG. 1a). In the latter approach, produced effluents are analyzed to construct a tracer production profile that is fitted to a convection-diffusion equation (CDE) for deriving a dispersion coefficient estimate (see FIG. 1b). Thus, both of these conventional approaches require keeping track (i.e., measuring) of tracer concentrations across the production outlet to derive the injected concentration profile of the tracer(s) for determining the dispersion coefficient and the dispersivity of the porous medium (e.g., a reservoir rock).